Treatment of heat transfer salts



May 15, 1945. F. c. FAHNESTOCK 2,375,760

TREATMENT OF HEAT TRANSFER SALTS Filed Jan. 9, 1942 k 7 v i g 4% f6 7 draw area/v map 7 47 sroxmss TANK INVENTOVR FRANK c, FA/I/VEJTOCK ATTORNEY Patented May 15, 1945 UNITED STATES 2,375,760 TREATMENT OF HEAT TRANSFER SALTS FranlrC. Fahnestock, Woodbury, N. 1., assignor to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application January 9, 1942, Serial No. 426,119

5 Claims.

The present invention relates to the art of temperature control, with particular reference to temperature control systems involving the use of heat transfer or thermophore salts.

It has heretofore been proposed to employ alkali metal salts of oxyacids of nitrogen, and in particular molten mixtures such as are produced by melting together potassium nitrate and sodium nitrite, for controlling the temperature of chemical equipment by indirect heat exchange, and such systems have now gone into wide usage. It has been found heretofore that the changes, occurring in the salt during use, result in an increase in the nitrate content with an accompanys ing decrease in nitrite and in the formation of caustic alkali. Otherwise, the salts have been believed to be satisfactory.

It has now been found that alkali carbonate is formed in addition to caustic alkali. The small; almost trivial amounts of alkali carbonate which are soluble do not substantially affect the melting point of the-salt, but it has also now been found that the presence of the carbonate undesirably affects the flow characteristics in the system, and it should be eliminated from the salts. The removalof the carbonate from the salt-in a commercially feasible manner is of substantial difliculty due to its low concentration.

Accordingly, objects of the present invention are to provide methods ofoperating temperature control systems employing molten thermophore salts in which the flow characteristics in the system are maintained by controlling within limits the concentration of carbonate within the system. A further object is to provide economically feasible methods of controlling the carbonate content of heat transfer salts. Other objects and advantages will be apparent from the following description and the accompanying drawing, which is a flow sheet of an application of the present invention. i

In accordance with the present invention, a

tween 0.5 and 3, and contain a percentage of nitrate ion between and 50% by weight. Salt, having a composition within this range, is withdrawn from the temperature control system and the melting point and temperature of the salt are reduced by the addition of water, either as low-pressure steam or liquid in amount between 10% and 30% of the total solution following addition. After dissolving the salt, an alkali earth salt of an oxyacid oi nitrogen is added, which effects precipitation. of alkali earth carbonate. The precipitate is separated at a temperature of at least 300 F. from the solution, which may necessitate heating the solution. Following removal of the precipitated carbonate the solution is concentrated to molten salt by heating till the salt contains not over 5% of water dissolved therein.

This final concentrating step has heretofore discouraged all investigation along the line of treating heat transfer salts in aqueous solution. The four salts, potassium nitrate and nitrite 'and sodium nitrate'and nitrite, which may 'be'employed in the preparation of salt mixtures the ions of which are potassium, sodium, nitrate and nitrite, have 'in common the following property.

If a solution of any of these salts is heated to drive oil the water, the salt is all deposited as solid phase prior to transition to molten salt. It has been expectedthat this would also occur with mixtures of the salts, since the property was common to all, and since solid phase is in fact deposited upon heating solutions of many mixtures of these salts. However, contrary to this expectation within the described range of composition, the transition is directly from solution to molten salt, with .no solid phase being formed.

As shown in the accompanying drawing, molten salt is maintained in a storage tank I. The salt may be withdrawn from the storage tank i through a dip tube and pipe 2 by a pump 3 and conveyed to a control zone 4 in which the salt is passed in indirect heat exchange relation with the materials the temperature of which it is desired to maintain. From the zone 4 the salt may be returned to the storage tank I by pipe 5. The circulating system involving storage tank ll, pump 3 and controlled zone 4 is representative of the general class of indirect heat exchange systems known in the art and involved in the present invention.

Salt is withdrawn from any desired location in the circulating system and is passed to treating tank 6 by valved line 1. Water or steam at low temperature is introduced into the withdrawn salt either in the treating tank 8 or in the line "I. Alternatively, the water may be placed in the tank 6 and the salt run into. the water. The water is added in amount to dilute the salt to a solution containing at least 10% water erably not over about 30%. Larger additions of water may be made but are disadvantageous since such larger amounts increase the cost of reconcentration. The water in addition to dissolving the salt reduces the temperature thereof considerably. Calcium nitrate or nitrite, or a mixture of the two, preferably in solution is then introduced into the salt in tank 6 in amount up to the stoichiometric equivalent of the carbonate present and preferably in amount sufficient that not over 0.3% carbonate (determined as umcon and prefcarbonate from such highly concentrated salt solutions without the use of excessive alkali earth salt.

The precipitatedalkali earth carbonate is then separated from the solution. In order to effect separation it has been found that a temperature of at least 10 F. above the initial crystallization temperature is necessary, and generally a temperature of about 300 F. or above is desirable. It the temperature has dropped below the desired level', the solution should be heated. Heating may be done in any desired manner depending upon available equipment and heat supply. As shown, heating may be efiected by high-pressure steam introduced through the perforated pipe.

8 in tank 5. The alkali earth carbonate whichi precipitates from these salt solutions is in a form which may be readily separated. The separation can be eil'ected by settling, filtration, or other suitable method. Thus, after the steam is shut off from pipe .8, the solution may be held in tank 8 until clear solution ,may be drawnpii from any desired level by one of a plurality of drains 9.

The salt solution is next concentrated to molten salt containing not over 5% water. The concentration can be eifected by running the solu- 80 tion into tank l0 provided with any suitable heater such as indirect steam heater II. The tank l0 if desired may be provi with a filter bed l2. The salt solution is with awn during concentration by .drain- I 4, provided with pump 80 I5, and is recirculated by valved return line It.- When the water content has been reduced to not more than 5%, the valve in line It is closed and the valve in line. I! is opened, whereby the salt now in molten state, i. e., containing not over 40 5% water, is returned to storage tank 1. Most I of the remaining water, the expulsion of which requires high temperature, is flashed ofl in the storage tank.

It operation oi. the settling or other initial separating step is carefully controlled, further filtering through bed i2 is unnecessary, but the secondary separation is customarily advisable as a safeguard against inaccurate control. Also with respect to primary separation by settling, it is to be noted that the precipitate and solution need not be removed after each batch treatment but the next batch or. molten salt and water maybe added directly thereto. After a considerable treatments, it may be withdrawn and the salt solution separated 'therefrom may be reconcentrated for use.

I claim as my invention:

1. The process of maintaining the flow char acteristics of a circulating indirect heat exchange system in which a heat transfer salt is utilized, which comprises employing a salt comprising principally potassium and sodium as the cations and n trate and nitrite as the anions in which the potassium to sodium weight ratio is between 0.5 and 3 and which contains between 15% and 50% nitrate ion by weight, withdrawing salt from the system, dissolving withdrawn salt to form an aqueous solution, adding an' alkali earth salt of an oxyacid of nitrogen, separating alkali earth carbonate precipitated thereby, and heating the solution to drive oil water, thereby eilecting transition to molten salt without formation of solid phase salt.

. 2. The process or maintaining the flow characteristics of an indirect heat exchange system in which a molten salt is employed which salt comprises principally potassium and sodium as the cations and nitrate and nitrite as the anions,

which process comprises withdrawing salt from the system, dissolving withdrawn salt to i'orm an aqueous solution containing between 10% and 30% water, adding a calcium salt of an oxyacid of nitrogen whereby to etl'ect precipitation of calcium carbonate, separating the precipitated calcium carbonate at a temperature of at least 300 F. from the solution, concentrating by heating to drive oil! water and form molten salt, the salt having a potassium to sodium weight ratio of between 0.5 and 3 and a nitrate ion content of between 15% and 50% by weight whereby the transition from solution to molten salt occurs without deposition of the salt as solid, and returning salt so treated to the system.

3. The process of eliminating dissolved carbonate from a molten heat transfersalt, which salt comprises principally potassium and sodium as the cations and nitrate and nitrite as the anions, comprising dissolving the salt in water in quantity suflicient to form a solution containing between 10% and 30% water, adding alkali earth salt'of an oxyacid obnitrogen to the solution whereby to effect precipitation or alkali earth carbonate, separating the precipitate at a temperature of at least 300 F., and concentrating the solution to molten salt, the salt being characterized by having a potassium to sodium weight ratio of between 0.5 and 3 and containing between l5% and- 50% nitrate ion by weight whereby the transition during concentrat'ng is directly from salt solution to molten saltwithout the deposition of the salt as solid.

4. The process of treating heat transfer salt. which" salt comprises principally potassium and sodium asthe cations and nitrate and nitrite as the anions, comprising dissolving thesalt in water sufllcient to produce a solution containing at least 10% water,'adding an alkali earth salt of an oxyacid of nitrogen to the solution whereby to effect precipitation of alkali earth carbon-;

ate, separating the precipitate from the solution, and concentrating the solution to molten salt, the salt being characterized by having a potassium to sodium weight ratio between 0.5 and 3 and having a nitrate ion content between 15% and 50% by weight whereby the transition durquantity of carbonate has formed by successive 55 comprises withdrawing used salt from said system', dissolving the withdrawn salt in water suiflcient to produce a solution containing at least 10% water, adding an alkali earth salt 01' an oxyacid or'nitrogen 'to the solution whereby to effect precipitation of alkali earth carbonate. separat'ng the precipitate from the solution, concen- 7i trating the solution from which the precipitate has been separated to molten salt, and returning said molten salt to said svstem. FRANK C. FAHNESTOCK. 

